Electronics Jam

This jam circuit can be used in
quiz contests wherein any participant
who presses his button
(switch) before the other contestants,
gets the first chance to answer a question.
The circuit given here permits up
to eight contestants with each one allotted
a distinct number (1 to 8). The
display will show the number of the contestant
pressing his button before the
others.

Simultaneously, a buzzer will
also sound. Both, the display as well as
the buzzer have to be reset manually
using a common reset switch.
Initially, when reset switch S9 is momentarily
pressed and released, all outputs
of 74LS373 (IC1) transparent latch
go ‘high’ since all the input data lines
are returned to Vcc via resistors R1
through R8. All eight outputs of IC1
are connected to inputs of priority encoder
74LS147 (IC2) as well as 8-input
NAND gate 74LS30 (IC3). The output
of IC3 thus becomes logic 0 which, after
inversion by NAND gate N2, is applied
to latch-enable pin 11 of IC1. With all
input pins of IC2 being logic 1, its BCD
output is 0000, which is applied to 7-
segment decoder/driver 74LS47 (IC6) after
inversion by hex inverter gates inside
74LS04 (IC5). Thus, on reset the
display shows 0.
When any one of the push-to-on
switches—S1 through S8—is pressed,
the corresponding output line of IC1 is
latched at logic 0 level and the display
indicates the number associated with
the specific switch. At the same time,
Electronic Jam
RAJESH K.P.
output pin 8 of IC3 becomes high, which
causes outputs of both gates

N1 and
N2 to go to logic 0 state. Logic 0 output
of gate N2 inhibits IC1, and thus pressing
of any other switch S1 through S8
has no effect. Thus, the contestant who
presses his switch first, jams the display
to show only his number. In the
unlikely event of simultaneous pressing
(within few nano-seconds difference)
of more than one switch, the higher
priority number (switch no.) will be
displayed. Simultaneously, the logic 0
output of gate N1 drives the buzzer via
pnp transistor BC158 (T1). The buzzer
as well the display can be reset (to
show 0) by momentary pressing of reset
switch S9 so that next round may
start.
Lab Note: The original circuit sent
by the author has been modified as it
did not jam the display, and a higher
number switch (higher priority), even
when pressed later, was able to change
the displayed number.